1. Field of the Invention
The present invention relates to a fixing apparatus installed in image forming apparatuses, copiers, and printers where an image is formed on a transfer material by an electrophotographic process, for fusing and fixing a development agent placed on the transfer material to it.
2. Description of the Related Art
In copiers and printers using an electron process, it is known that a toner image formed on a photoconductive drum is transferred onto a transfer material and thereafter the toner image is fused in a fixing apparatus having a heating roller and a pressurizing roller and fixed onto the transfer material.
At this time, for controlling the temperature of the heating roller, a method of detecting the surface temperature of the heating roller by a detecting element, which is arranged in contact with the surface of the heating roller, has been known. However, the contact-type temperature detecting element slides on the surface of the heating roller, so that it may degrade the surface, thereby shortening the life of the heating roller. When the surface is degraded, the sensitivity of the detecting element decreases, with the result that the wrong temperature may be detected.
Also another method has been known which uses a temperature detecting element for detecting the temperature of a heating roller without being in contact with the heating roller by sensing infrared rays emitted from the heating roller.
However, the emission rate of infrared rays from a heating roller detected by the non contact temperature detecting element differs between the life-start time and the life-end time since the surface of the heating roller is gradually degraded while being in contact with a transfer material holding toner thereon. Since the degradation of the surface of the heating roller varies depending upon the type and size of a transfer material to be fed, the infrared ray emission rate varies in the longitudinal direction of the roller. To describe more specifically, since infrared ray emission rate changes, the time for the temperature detected by the non-contact temperature-detecting element to reach a predetermined temperature delays.
For example, Japanese Patent Application KOKAI Nos. 2001-242743 or 2000-259033 discloses a fixing apparatus in which the temperature of a heating roller is detected by a temperature-detecting element in contact with a non-paper-feeding area and a non-contact temperature-detecting element for detecting the temperature of a paper-feeding area. However, a time lag is produced since the sensitivity differs between the contact-type temperature detector and the non-contact temperature detector. Hence, it has been difficult to accurately detect the temperature of a heating roller.
Furthermore, Japanese Patent Application KOKAI No. 2000-227732 discloses a fixing apparatus in which the radiation emission rate from a heating roller installed therein is detected by a radiation-detecting unit. However, since the radiation-detecting device is installed in a fixing apparatus where toner and paper dust are scattered, wrong detection may occur due to smudges. To solve this, it is effective to provide not only cleaning means to the temperature detecting element but also cleaning means to the radiation detecting unit; however they increase cost. In addition, the infrared ray emission rate varies depending upon how significantly stained on the surface of the heating roller.
Furthermore, in Japanese Patent Application KOKAI No. 2001-34109 discloses a technique for correcting temperature based on the difference between the temperature detected by an infrared detecting member for detecting the amount of infrared rays and the temperature detected by surface temperature detecting means which includes a thermistor for correcting the temperature of the infrared detecting member.
In this way, in the case where a non-contact temperature-detecting element is used, when infrared ray radiation rate changes, the temperature of the heating roller cannot be sensed accurately. Since heating means for heating the heating roller control heat generation depending upon the surface temperature of the heating roller inaccurately detected, the heating roller is not controlled at a proper temperature.